David J. Craig wrote:
There are several cards that provide a power connection. Macally has
one that I use that also has 6 1394 ports. Drawing that much power from
the PCI bus is not wise nor good for stability. I wish all cards would
do this for 1394 and USB. Even 0.5 amp per port for USB can be a lot if
you have 8 USB ports. The 1394 has a problem in that the voltage is not
fixed, but allowed to be variable - probably not when running, but
different cards can output different voltages. Don’t know why they did
it that way, but it is interesting.
Thanks, always happy to mark another source.
IEEE 1394a defines power on the cable to be 8V (I think) - 33V
unregulated. That means that the voltage does not have to be kept
constant. That simplifies things greatly and makes things more
flexible from the point of view of the power suppliers but, of course,
makes things more complex from the point of view of the power
consumer which must have the appropriate circuitry to generate the
voltages required. Note that the 1394 spec. allows the bus to carry
quite a bit of power which is much easier to do in an unregulated
manner.